The (electro)-catalytic decomposition of formic acid in solution is a key reaction for transfer hydrogenation, hydrogen storage and direct formic acid fuel cell.1) On the one hand, our first principles simulations of the electrooxidation of formic acid over nickel identify the reorientation of the formate intermediate and the desorption of CO2 as the rate-limiting steps.2) Although they are not associated with an electron transfer, these barriers are strongly modified when the electrochemical potential is explicitly accounted for using a surface charging method and when modelling the influence of the solvent with a continuum. Hence, such a level of modelling is key to understand the kinetic limitations that penalize the reaction. On the other hand, in heterogenous catalytic conditions, palladium supported catalysts are most promising, but the efficiency and selectivity are strongly impacted by the pH, or in other words by the presence of sodium formate in solution.3) We will show that the presence of this anionic promoter is key to render palladium really active. The promoter effect is traced back to the modulation of the electric field at the catalyst surface, with a strongly contrasted action on the various species along the competing pathways. Here again, the activation barrier for the reorientation of the formate intermediate is strongly modified. The promoter effect is well captured by modelling the anion by an electron injected into the surface, and an idealized counter-ion distribution representative of the electrolyte, in a methodology that is reminiscent of the modelling of electrocatalytic processes. However, we will show that the chemical promotion through an adsorbate investigated herein is complementary to electrocatalytic conditions where the potential is maintained constant. References 1.Q.-L. Zhu, Q. Xu, Energ. Envir. Sci. 2015, 8, 478-512 2.S.N. Steinmann, C. Michel, R. Schwiedernoch, J.-S. Filhol, P. Sautet, , ChemPhysChem, 2015, 16, 2307-2311 3.K. Jiang, K. Xu, S. Zou, W.-B. Cai, J. Am. Chem. Soc. 2014, 136, 4861.